Boron-Nitrogen Dative Bonds in Structural Supramolecular Chemistry

A low optical bandgap and fullerene-like electron affinity of functionalized cyclopenta[hi]aceanthrylene (cypac) was reported and potential applications as electron acceptors in organic solar cells are suggested among others.[1] In this work, a water-soluble, bolaamphiphilic cypac derivative with a low-lying LUMO and intense absorption bands over an extended range of the UV-visible spectral region has been synthesized.[2] Self-assembly of cypac-bis-phosphate in aqueous medium leads to the formation of a supramolecular polymer with a vesicular morphology. The electronic properties of the monomer remain largely intact upon supramolecular polymer formation, as evidenced by UV-vis and cyclic voltammetry measurements.

Driven by the concern about the future petro-chemical feedstock availability, as well as the need for a green and sustainable society, renewable and bio-based materials have become the natural candidates for alternatives to petro-chemicals. As the most abundant biomass feedstock, carbohydrates are attracting significant attention from both academia and industry for the production of chemicals, fuels and energy. However, one major drawback of directly using carbohydrates in industrial processes is their limited thermal stability caused by the existence of multiple functional groups, for instance hydroxyl groups. Consequently, simplified or bi-functional carbohydrate-derived building blocks are often more attractive than the virgin carbohydrates for polymer chemists. In this thesis, we aim to 1) develop a new family of difunctional bio-based building blocks, and 2) utilize these new building blocks as AA monomers to synthesize step-growth polymers, such as polyesters and polyamides. Ideally, the new polymers are expected to be semicrystalline, preferably having high melting (Tm) and high glass transition temperatures (Tg), and can be used as the potential fully or partially bio-based engineering plastics. Isohexides (1,3:4,6-dianhydrohexitols), a group of secondary, rigid diols, as well as their derivatives are interesting carbohydrate-derived building blocks for various types of step-growth polymers, such as polyesters, polyamides and polycarbonates. Due to the intrinsic rigidity of the structures, isohexides are capable of increasing the glass transition temperatures when incorporated into polymers. However, the relatively poor reactivity of the secondary hydroxyl groups of isohexides has been recognized as a main drawback, which often results in low molecular weight or discolored polymers. In order to overcome these problems, as well as utilizing the rigidity of the isohexide skeleton, a new family of 1-carbon extended isohexide-derivatives were designed and synthesized by transforming the less reactive secondary hydroxyl functionalities into more reactive ones. Based on a key dinitrile intermediate, viz. isoididedinitrile (IIDN), the novel isohexide-derived building blocks were obtained. They are: isoidide-2,5-dicarboxylic acid (IIDCA) and its dimethyl ester (IIDMC), isoidide-2,5-dimethanol (IIDML) and isoidide-2,5-dimethyleneamine (IIDMA). After optimization of the respective synthetic routes, these building blocks were obtained at sufficient scales (5–20 g/batch) and high purities (>99.5%) to be suitable for polymerizations. Three series of polyesters based on the novel 1-carbon extended isohexides were synthesized. The first series of polyesters is based on isoidide dicarboxylic acid (IIDCA) and linear alkylene diols. Unexpectedly, they have rather low Tm (60–100 °C) values whereas the recorded low Tg values (-30–18 °C) were approximately expected. The second series of polyesters is fully based on renewable, isohexide-derived monomers. The combination of both isohexide-based AA/BB monomers gave significantly higher Tg values (45–85 °C) than the partially linear aliphatic polyesters of the first series. Importantly, this part of the project revealed the promising characteristics (reactivity, rigidity and thermal stability) of isoidide-2,5-dimethanol (IIDML) for potential applications in performance polymers with high Tg and Tm. Therefore, the third series of polyesters was entirely focused on IIDML-based semi-aromatic systems. The Tg and Tm values of these polyesters were found to exceed even those of the conventional PET and PBT polyesters, confirming our expectation that IIDML is a promising rigid cyclic diol for engineering plastic applications. Two series of semicrystalline (co)polyamides were synthesized from the new isoidide 2,5-dimethyleneamine (IIDMA) or from a mixture of IIDMA and 1,6-hexamethylene diamine with the biobased dicarboxylic acids sebacic acid or brassylic acid. A combination of melt polymerization of nylon salts with solid-state polycondensation (SSPC) processes afforded the desired copolyamides with maximum number average molecular weight (Mn) of 49,000 g/mol. Analysis by temperature dependent soldi state NMR and FT-IR spectroscopies revealed that the new diamine IIDMA exist in both the crystalline and amorphous phases of the copolyamides. Given the sufficiently high Tm values (~200 °C) of the copolyamides containing less than 50% of IIDMA, these bio-based semicrystalline copolyamides can be useful for engineering plastic applications. By comparing the thermal properties of the polymers based on the new isohexide-based building blocks, i.e. IIDCA, IIDML and IIDMA, with relevant reference polymers, systematic structure-thermal properties studies of each series of polymers were conducted. This work objectively evaluated the relative rigidities, reactivities and thermal stabilities of these new building blocks with respect to those of the parent isohexides, as well as with respect to their linear, cyclic aliphatic and aromatic analogues. Both the experimental (i.e., single crystal X-ray diffraction) and computational-aided methods (i.e., Density Functional Theory (DFT)) were employed to elucidate the structural characteristics of the isohexide-based building blocks, as well as to understand the specific thermal properties of the polymers based thereon. Additional efforts were invested to expand the dinitrile platform to the other stereo-isomers. The isomeric isohexide dinitriles were synthesized by base-induced epimerization. The strong non-nucleophilic organic base 1,8-diazabicycloundec-7-ene (DBU) was found to be a suitable catalyst for such type of isomerization. A kinetic study on the epimerization of three dinitrile isomers revealed an equilibrium between the three possible isomers in THF. Single crystal X-ray diffraction studies demonstrated several structural characters of these molecules regarding their symmetry and linearity. Density Functional Theory (DFT) calculations further supported the base-induced epimerization mechanism. This work concerns a pioneering study of deoxy-1-carbon-extended isohexide-based building blocks. All the monomers and polymers synthesized, characterized and evaluated in this thesis are entirely new. Suitable synthetic laboratory-scale protocols were developed for all the monomers and polymers. The reported study of the structure-thermal properties relations, as well as the crystal structure elucidation by single crystal X-ray diffraction and DFT, are also of high importance concerning the field of isohexide-based polymers and will certainly prove to be very useful for drawing conclusions on the industrial feasibility of such bio-based materials.

Open AccessCCS ChemistryMINI REVIEW1 Mar 2022Multidimensional Mass Spectrometry Assisted Metallo-Supramolecular Chemistry Heng Wang, Chenxing Guo and Xiaopeng Li Heng Wang College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 Shenzhen University General Hospital, Shenzhen University Clinical Medical Academy, Shenzhen 518055 , Chenxing Guo College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 and Xiaopeng Li *Corresponding author: E-mail Address: [email protected] College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 Shenzhen University General Hospital, Shenzhen University Clinical Medical Academy, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202101408 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metallo-supramolecular chemistry based on the reversible, directional, and predictable noncovalent interaction of metal–ligand coordination has been widely applied in constructing a myriad of two- and three-dimensional supramolecules with sophisticated structures. The development of metallo-supramolecular chemistry highly relies on facile characterization methods. For advancement of structural complexity with desired functions, however, conventional crystallographic and spectroscopic characterizations are limited by crystal growth, sample purity, and severe signal overlaps. With high sensitivity and low requirement for purity, multidimensional mass spectrometry (MS), including electrospray ionization-MS (ESI-MS), tandem MS (MS2), and ion mobility-MS (IM-MS), is emerging as an essential analytical tool for characterization of the chemical compositions, connectivity, stability, shape, and size of metallo-supramolecules, as well as kinetic and thermodynamic features of self-assembly processes. Thus, this review mainly focuses on the recent development of multidimensional MS in metallo-supramolecular characterization and discusses how ESI-MS, MS2, and IM-MS assist the development of metallo-supramolecular chemistry. Download figure Download PowerPoint Introduction Supramolecular self-assembly, inspired by the precise organization of bio-macromolecules in living systems, allows for the spontaneous formation of ordered complexes from a chaos state.1 Among various noncovalent driving forces, metal–ligand coordination plays a vital role in the self-assembly of structurally and functionally sophisticated metallo-supramolecules, benefitting from the high directionality and predictable feature of coordination bonds.2–6 As a result, metallo-supramolecular chemistry has witnessed an explosion in constructing a myriad of artificial supramolecular architectures, including polygons,7–11 knots,12–15 links,16–19 fractals,20–22 as well as Platonic,23–27 Archimedean,28–34 Goldenberg polyhedrons,35,36 and so on. And these complexes show great potentials among different fields, including catalysis,37–46 nano-electronics,47,48 stabilization of air- or water-sensitive molecules,49–51 disease diagnosis and therapy,52–57 and so on. Furthermore, various delicate assembling approaches, inter alia, directional bonding,2 symmetry interactions,58 molecular paneling,59 and sub-components,60 have been developed to assist the exploration of metallo-supramolecular chemistry. Compared to the flourish of self-assembly strategies, however, the lack of efficient characterization methods always limits the investigation of assembled complexes at equilibrium and the kinetic details of self-assembly processes in solution because of the weak interaction and dynamic nature of metallo-supramolecules. Among the conventional characterizations of metallo-supramolecules, single-crystal X-ray diffraction (SC-XRD) is a powerful and direct method that allows for detailed structural information at the atomic level.61 However, it remains a formidable challenge to obtain high-quality single crystals, particularly for metallo-supramolecules with large voids filled by disordered solvent molecules or counterions. In addition, the refinement of the crystal structures for large metallo-supramolecules is difficult because the current crystal refinement approaches were mainly developed for small molecules. Apart from those issues the structural information obtained through SC-XRD in solid phase may not correlate with the study of metallo-supramolecular systems in solution or gas phase. Another well-established characterization method is NMR spectroscopy, whereby various nuclei in metallo-supramolecular systems, such as 1H, 13C, 19F, 31P, and so on can be detected, allowing the chemical environment of these nuclei in metallo-supramolecules to be elucidated.62,63 Through two-dimensional (2D) NMR spectroscopic techniques, for example, 1H–1H correlation spectroscopy (COSY), nuclear Overhauser effect spectroscopy (NOESY), and diffusion ordered spectroscopy (DOSY), diverse structural information about the metallo-supramolecules can be further obtained, including metal–ligand coordinations, geometric distributions of nuclei, and details of the size and shape of assembled architectures. However, the application of NMR is also limited in analyzing samples with low purity, severe overlapping signals, or those containing paramagnetic metal ions. As complementary characterization techniques, various microscopic methods, including atomic force microscopy (AFM), scanning tunneling microscopy (STM), and transmission electron microscopy (TEM), directly image the analytes. AFM and STM, in which tips probe the surface of the target molecules on substrates, reveal the size and shape of an individual molecule64 and distinguish the metal ions by using scanning tunneling spectroscopy (STS).65 However, high image resolution relies on sharp tips, as well as high-quality samples without impurities, severe aggregations, or dissociations of the target supramolecules. TEM uses electrons as the "probe" and can offer atomic resolution of the entire molecular structure assisted by computer-assisted three-dimensional (3D) reconstruction, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) techniques.66 Unfortunately, in addition to the challenges of preparing a high-quality sample, TEM also suffers from the intrinsic instability of metallo-supramolecules under electron beam or during sample preparation. The mass-to-charge ratio (m/z) information of the intact metallo-supramolecular ions obtained by mass spectrometry (MS) measurements in the gas phase enables the determination of the chemical composition of analytes. Furthermore, MS characterization provides higher sensitivity but a lower sample purity requirement than crystallographic analysis and NMR. Such features along with high analytical speed make it possible to monitor the dynamic process of self-assembly before the system in question reaches equilibrium. Thus, MS is emerging as an essential analytical tool for determining structures and monitoring reaction processes. However, traditional MS approaches also suffer from the fragmentation of metallo-supramolecules bound by weak coordination interaction even when a soft ionization method, that is, electrospray ionization (ESI), is applied. Cold-spray ionization (CSI),67 equipped with an ESI source cooled by liquid nitrogen, was used to overcome the problem by lowering the internal energy gained during the ionization. Due to the labile connectivity, matrix-assisted laser desorption/ionization (MALDI) could cause very severe fragmentation and thus is not preferred for the characterization of metallo-supramolecules. In addition to the m/z information obtained from MS, tandem MS (MS2) and ion mobility-MS (IM-MS) can elucidate more detailed information of the analytes. First, as a controllable fragmentation technique, MS2 offers deeper insight into the analytes, such as the information about the stability and connectivity of the structures. Furthermore, as an orthogonal in situ separation method, IM-MS can identify different species with the same m/z; more importantly, it provides structural information in terms of the shape and size of selected ions. During the past two decades, multidimensional MS with the combination of MS, IM-MS, and MS2 has played an important role in the development of metallo-supramolecular chemistry. Here, we discuss how ESI-MS, MS2, and IM-MS can be applied in (1) determining chemical compositions, connectivity, shape, and size of metallo-supramolecules, (2) distinguishing the isomers and by-products present in a dynamic combinatorial library, and (3) investigating kinetic and thermodynamic features of self-assembly processes. More importantly, we anticipate that multidimensional MS would not only act as an analytical method, but also serve as a powerful tool to assist the design of next-generation metallo-supramolecules, and even advance the development of new synthetic strategies. ESI-MS Studies Charged coordination complexes are ideal analytes for ESI-MS studies because of the easy-to-ionize feature of these supramolecules in which the counterions are readily stripped off, especially in polar and volatile solvents. Another advantage of ESI-MS is its intrinsic capability of reducing the signals corresponding to the multiply charged analytes with high molar mass (<104 Da) to the mass range of the conventional mass analyzers, since it only measures the m/z values of these ensembles. Consequently, a series of peaks is usually observed, representing different charge states of the same intact analyte through successive loss of counterions. To minimize the fragmentation during ionization, it is crucial to ionize the fragile analytes under mild conditions, including careful selection of the focusing and ionization voltages, low operation temperature, and fast ion extraction process through increasing source pressure (or a low vacuum).68 Additionally, highly concentrated metallo-supramolecular samples are often required to inhibit the potential entropy-driven dissociation in solution, although the ESI source needs more extensive cleaning after each measurement. Without annoying fragmentation, ESI-MS is suitable for detecting the chemical compositions of the ionic species in the metallo-supramolecular analytes, as well as providing information regarding the following aspects of the self-assembly: (1) fast screening the assembled structures; (2) monitoring the dynamic processes during the self-assembly; and (3) detecting the minor products to guide further design. Acting as a fast-screening characterization approach for the self-assembly Fast screening of the analytes' structural information by ESI-MS does not require extensive purification processes that are otherwise inevitable for NMR measurement and single-crystal growth. In most cases, after simply removing the inorganic salts to reduce ion suppression, the resulting sample solution is ready for direct measurement. With careful control of stoichiometry, the solution containing the self-assembled species can be directly subjected to ESI-MS without any further purification. Additionally, its capability of detecting m/z values enables the characterization of multiple assemblies with different numbers of building blocks, paramagnetic metallo-supramolecular analytes, and host–guest adducts. Therefore, ESI-MS could serve as a complementary or even superior method to perform structural characterization compared to NMR. The first example showcasing the fast screening by ESI-MS is the characterization of different generations of metallo-dendrimers S1/ S2 (Figure 1a) comprised of polyazaaromatic Ru(II) complexes reported by Kirsch-De Mesmaeker et al.69 ESI-MS directly generated clear and simple patterns that allowed the unambiguous determination of the chemical formula, whereas the other spectroscopic characterization just provided nonconclusive data (Figure 1b). Second, in the characterization of metallo-macrocycles reported by Newkome and co-workers,701H NMR gave very similar chemical shifts for the hexagon, octagon, and decagon ( S3- S5) assembled from the Ru(II)-connected bitopic 2,2′:6′,2″-terpyridyl (TPY) subunit L3 (Figure 1c) because of the similar symmetry and chemical environments of the detected protons. In contrast, these analogs with distinct molar mass were unambiguously identified by ESI-MS, where three distinct sets of peaks were detected (Figures 1d and 1e).70 Figure 1 | ESI-MS acting as a fast-screening approach for the preparation of (a and b) metallo-dendrimers comprised of polyazaaromatic Ru(II) complexes S2 and its corresponding ESI-MS spectrum; (c–e) TPY-Ru(II)/Fe(II)-based metallo-hexagon S3, -octagon S4, and -decagon S5 with corresponding 1H NMR spectra and ESI-MS spectra; (f–h) self-assembly of supramolecular fractals S6 based on the metal complexation between a TPY-based ligand and paramagnetic metal ions with corresponding 1H NMR and ESI-MS spectrum of the Co(II) complex (S6-Co). (a and b) Adapted with permission from ref 69. Copyright 1996 American Chemical Society. (c–e) Adapted with permission from ref 70. Copyright 2011 Wiley-VCH. (f–h) Adapted with permission from ref 71. Copyright 2020 American Chemical Society. Download figure Download PowerPoint Metallo-supramolecules containing paramagnetic transition metal ions with unpaired electrons remain a formidable challenge for NMR characterization. In contrast, ESI-MS has no problem characterizing these samples. For example, Shionoya et al.25 constructed a series of metallo-supramolecular octahedra by mixing a tritopic pyridinyl building block with different transition metal ions. ESI-MS spectra of the paramagnetic analytes containing Cu(II), Ni(II), Co(II), and Mn(II) ions clearly showed a dominant set of peaks ascribed to the designed structures; whereas, NMR spectra only displayed broad peaks with limited information.25 Similarly, with the help of ESI-MS characterization, coordination-driven self-assembly based on a TPY-paramagnetic metal ion complex was applied to construct a series of supramolecular fractals reported by Li et al.,71 which not only expanded the library of metal ions for assembling TPY-based architectures but also set up an ideal platform for studying the kinetic and thermodynamic features of these metallo-supramolecules (Figures 1f–1h). When characterizing the paramagnetic Co(II) and Ni(II) cages assembled using the sub-component self-assembly strategy, the reliable ESI-MS method also played a vital role in identifying the formation of target structures.72,73 Investigation of host–guest complexation is an important subfield of metallo-supramolecular chemistry, which is related to the fundamental study of molecular recognition and the potential applications of these metal–organic containers. NMR spectroscopy is extensively applied to provide evidence for the interaction with diagnostic chemical shifts of the protons on guest molecules. However, it cannot always accurately address how many guest molecules are encapsulated in the containers. ESI-MS can properly examine the exact change in mass upon the encapsulation of the guest molecules, thus, providing an accurate m/z value for deducing the number of guest molecules. In a self-assembled M8L6 (M = the metal ion, L = the ligand) cube with porphyrin faces, Nitschke et al.74 described that large guests such as coronene and fullerene can be selectively encapsulated (Figure 2a). Compared with the complicated NMR spectra that only provided a 2:1 ratio of guests in two different environments, ESI-MS spectrum of the coronene-encapsulated cube directly provided the number of the guest molecules via deconvolution of the dominant set of peaks (Figures 2b and 2c). The ESI-MS spectrum also clearly confirmed that the cube allowed for selective encapsulation of C70, C76, C82, and C84 over C60 from the fullerene soot, as evidenced by the absence of any discernible peaks assignable to the C60-encapsulated or guest-free cube. Figure 2 | ESI-MS-based fast screening for studying the guest encapsulation of metallo-supramolecular cages. (a–c) Self-assembly of porphyrin-faced cube S8 for encapsulation of three coronene with corresponding characterization by 1H NMR and ESI-MS; (d and e) a PBI-edged tetrahedron capable of encapsulating two molecules of C60S9 and the corresponding ESI-MS spectrum; (f–h) self-assembly of a metallo-supramolecular peanut S10 for simultaneous encapsulation of one diamantane and two phenanthrenes (S11) with corresponding characterization by 1H NMR and ESI-MS. (a–c) Adapted with permission from ref 74. Copyright 2011 Wiley-VCH. (d and e) Adapted with permission from ref 76. Copyright 2013 American Chemical Society. (f–h) Adapted with permission from ref 83. Copyright 2017 Springer Nature. Download figure Download PowerPoint Thus, ESI-MS is also a feasible and rapid method for qualitatively studying the encapsulation selectivity of a certain metallo-supramolecule with different guests. In another example of selective encapsulation, when mixing a metallo-supramolecular tetragonal prism with a mixture of C60 and its derivatives, the selectivity could be determined qualitatively by comparing the relative abundance of the peaks corresponding to the host–guest complexes containing different guests. Among them, C60 and PCBM-C60 have higher affinity with the host than N-methylpyrrolidine-C60.75 However, it is still challenging to quantitatively determine the selectivity by ESI-MS because of the different ionization efficiency of those complexes, although the host molecules are the same. Ideally, quantitative data could be obtained if the ionization efficiencies of different complexes are calibrated. In addition to encapsulating one fullerene molecule, Würthner et al.76 applied ESI-MS to unequivocally prove the encapsulation of one or two molecules of C60 inside the cavity of perylene-bisimide (PBI) edged tetrahedron S9 (Figures 2d and 2e). Note that the C60 guest does not bear any protons so that it can only be identified by 13C NMR. Although the 13C NMR could confirm the encapsulation of the C60, it was unable to determine the number of guest molecules because conventional 13C NMR does not allow for quantitative analysis as opposed to 1H NMR. Metallo-supramolecular trigonal prisms, wherein C3-symmetrical tritopic pyridyl ligands, linear ditopic pyridyl ligands, and an end-capped Pd(II) moiety served as the base faces, pillars, and vertices, respectively, were found to be an excellent molecular container capable of encapsulating a variety of guest molecules, including, coronene,77 triphenylene,78 pyrene-4,5-dione,79,80 and metal–organic complexes.81,82 More intriguingly, the number of the encapsulated guest molecules can be precisely manipulated via tuning the length of the organic pillars. The accurate number of encapsulated guests could be unambiguously determined by ESI-MS. A more complicated situation was reported by Yoshizawa et al.83 The self-assembled metallo-supramolecular peanut S10 heteroleptically encapsulated two different types of guests, that is, one diamantane and two phenanthrenes, inside its two cavities (Figure 2f). Without an SC-XRD result or clear NMR spectrum (Figure 2g), such an unusual encapsulation was identified by ESI-MS (Figure 2h), where peaks with m/z values of the expected host–guest complex were found. Monitoring the dynamic processes for unveiling self-assembly details Metallo-supramolecules bonded via labile coordination interactions exhibit dynamic features in solution due to the dissociation and reassociation of the weak bonds. Thus, it is hard to investigate the kinetic processes by traditional characterization techniques in solution. Nevertheless, in an ESI-MS measurement, each complex ion is a separated species in gas phase. Therefore, the dynamic process is "frozen" to display the kinetic instead of thermodynamic feature of the system. As one type of dynamic process, ligand exchange involves the disassociation of a ligand from the assembled supramolecule and subsequent replacement by another ligand. Such a process is related to the coordination bond strength and the activation energies during the exchange processes. If the time scales of such dynamic exchanges are comparable for MS measurement, one should be able to study the exchange process of two kinds of pre-assembled supramolecules with distinct molar masses of ligands by MS. Once the exchange occurs, the m/z values corresponding to each possible re-assembled species as well as the initial ensembles would be different, all of which can be specifically recorded by ESI-MS measurement. For example, Fujita et al.84 reported the studies of ligand exchange between two M12L24 spheres via mixing two topologically identical spheres bearing different length of alkyl chains (–OC3H7 vs –OC6H13) and then monitoring under time-dependent ESI-MS (Figures 3a and 3b). Due to the difference in mass, all the peaks corresponding to the initial two spheres and the exchanged ones were easily identified in the spectra. Time-dependent measurements revealed a quite remarkable kinetic stability of the spheres regarding ligand exchange. The half-time for the first ligand exchange was measured to be approximately 20 days. After evaluating the ionization efficiency of two spherical supramolecules, the estimated kinetic ligand exchange rate was found to be 2 × 10−2 M−1·s−1, which is far less than the exchange rate between a free ligand with a sphere (half-time of 23 min). Therefore, the ESI-MS studies semi-quantitatively revealed that the fully closed framework slowed down the dynamic exchange processes through a cooperative effect.84 A similar method was also utilized to carefully study the dynamic nature of metallo-polygons assembled from organoplatinum motifs and pyridyl ligands with 1H/2D isotope labeling.85 In addition to the quantitative determination of the ligand exchange rates, further understanding about the kinetic processes in coordination-driven self-assembly was provided by time-dependent ESI-MS, which showed the ligand exchange rate highly depended on the experimental conditions such as temperature, solvent, and counterions. Figure 3 | ESI-MS of ligand exchange. (a) Ligand exchange between two structurally identical spheres (Pd12L724 and Pd12L824) with different alkyl chain lengths (-n-C3H7 vs -n-C6H13), and (b) the time-dependent ESI-MS spectra; (c) self-assembly of four types of supramolecular snowflakes (S12–S15) with different lengths of the ligands, and time-dependent ESI-MS spectra of the mixture of and (a and b) Adapted with permission from ref Copyright American Chemical Society. and e) with permission from ref Copyright Wiley-VCH. with permission from ref Copyright 2017 American Chemical Society. Download figure Download PowerPoint In addition to the pyridyl ESI-MS also provided insight into the ligand exchange processes of metallo-supramolecular systems based on analyzing the peaks to the complexes in ESI-MS it was revealed that the exchange processes a certain of to the size and shape of the ligand via the of the structure when the Through a time-dependent ESI-MS measurement, such a was estimated in the metallo-supramolecular snowflakes with different length of ligand The exchange rate was found to directly correlate with that is, the the the ligand exchange. When = the exchange equilibrium = required for whereas = in no exchange (Figures Furthermore, the ligand exchange rate was also found to correlate with the coordination metal ion in TPY-based To quantitatively these exchange rates, different metal ions (M = were applied. of these metal ions was used to two structures with the same molecular size and shape but different molar After mixing the of pre-assembled supramolecules, time-dependent ESI-MS was utilized to the relative abundance of each After the ionization efficiency of these supramolecules, ESI-MS data provided exchange of these metallo-supramolecules at with a of Mn(II) Co(II), whereas the and fractals were found to be to the ligand exchange. The exchange rate of fractals was determined to be × M−1·s−1, which is higher than that of fractals × Monitoring the in the self-assembly processes is also a kinetic investigation that can be carefully by ESI-MS. When using NMR to monitor the complexation of ligands with or Ni(II) shifts of the peaks were observed, the of an before the formation of However, the limited information provided by NMR spectra not elucidate the ESI-MS was then used to the and the that is, linear by the m/z value of the intact Thus, ESI-MS can reveal the reaction via monitoring the Such capability was also used in monitoring the formation of a supramolecular wherein ligand and self-assembly were in a The time-dependent the of the coordination and processes (Figures and Figure | (a) The formation of supramolecular via a ligand and self-assembly (b) time-dependent ESI-MS monitoring the (b) Adapted with permission from ref Copyright American Chemical Society. Download figure Download PowerPoint With m/z values of the more details about the self-assembly processes could be allowing for deeper understanding of the assembling For in the of ligands with ions to and an with metal ion in the ligand with instead of was through ESI-MS by increasing ligand or metal Such an that the complexation to the A similar of complexation was also in other systems by ESI-MS, its ESI-MS of the species during the formation of prism with a ligand revealed the of the The ESI spectra the of self-assembly and showed the formation of the base with a Such a the design of a building block with two types of two distinct geometric that is, as base and respectively, allowing for the self-assembly of a structures to further design With the reliable m/z ESI-MS can identify the desired before other and characterization. For based on the directional an tritopic ligand was designed to metallo-supramolecules by coordination with or to the assembled could be or Unfortunately, NMR characterization because of the Nevertheless, ESI-MS provided evidence for the formation of a which the of the ligand to the of the complex for structural on

Exciton-polarons in self-assembling helical aggregates: relating optical properties to supramolecular structure

Conventional amphiphilic block copolymers are macromolecules consisting of at least a hydrophilic segment covalently attached to a hydrophobic segment. They are unique and versatile building blocks in supramolecular polymer chemistry, both for the generation of highly organized, self-assembled structures and for the structural control of material interfaces. In the absence of solvents, the phase-behavior of block-copolymers is sparingly described. In aqueous solutions block copolymers self-assemble into nanoscopic objects. These structures are gaining more and more attention for technical formulations. Self-assembly is simply induced by dissolving dry powder of amphiphilic copolymers in water. This self-assembly process, however, cannot be described solely based on bulk phase behavior and very few experimental phase diagrams of block copolymer/water mixtures have been reported. In order to control, predict and eventually achieve a comprehensive understanding of this process and in particular the formation of vesicles, worm/rod-like micelles and spherical micelles it is important to systematically investigate the phase behavior of block copolymers across the whole water concentration range. We therefore undertook investigations of the self-assembly of two block copolymers. Preliminary encapsulation studies reveal their potential for applications relevant for drug delivery. &#13;\nThe concentration profile of the identified amphiphilic block-copolymers diluted in aqueous solution was examined across the whole concentration range with four poly(ethylene oxide)-block-poly(γ-methyl-ε-caprolactone) (PEO-b-PMCL) block copolymers keeping the hydrophilic block length constant but varying the hydrophobic lengths. Bulk polymers did not display any ordered morphology. With increasing water concentrations the polymers underwent transitions from lamellar phases to packed vesicles and eventually all polymers self-assembled into vesicles in dilute aqueous solutions. At high concentration the largest polymer organized further into an inverse hexagonal phase prior to self-assembly into a lamellar phase. The smallest block-copolymer also self-aggregated into rod-like micelles and exhibited a hexagonal phase between the packing of the rod-like micelles/vesicles and the lamellar phase.&#13;\nThe concentration profile of phases assembled by amphiphilic block-copolymers diluted in aqueous solution was examined as well across the whole concentration range with three poly(isobutylene)-block-poly(ethylene oxide) block copolymers having the same hydrophobic block length but varying in their hydrophilic lengths. In contrast to PEO-b-PMCL these polymers self-aggregated depending on the hydrophilic length into micelles, worm-like micelles and for the shortest polymer into vesicles. Thus, increasing the polymer concentration lead to a different phase behavior. The largest polymer which solely formed micelles, underwent transitions from packed micelles with a few packed worm-like micelles to a hexagonal phase and finally into a lamellar phase, in contrary to the middle length polymer, which self-aggregated into worm-like micelles. The best to our knowledge, it is the first time that an experimental phase diagram is built such that it displays the formation of worm/rod-like micelles. With the composition studied in here, the worms, with increasing polymer concentration, pack and undergo afterwards into a hexagonal and finally a lamellar phase has been observed. For the shortest polymer the phase behavior was similar to that of vesicles self-assembled by PEO-b-PMCL. In contrast, the pure polymer formed an inverse hexagonal phase.&#13;\nIt was eventually shown that PEO23-PMCl32 can be used to encapsulate both small and large molecules. By adjusting the pH towards acidic conditions the responsiveness of the vesicles could be triggered to release the encapsulated material.

Pagoda[5]arene (P5), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at 1,5 positions, was conveniently synthesized in 43% by trifluoroacetic acid ...

The self-assembly of 3,6-disubstituted amine-linked phenanthrene trimers with a positively charged backbone leads to the formation of vesicles in aqueous medium. Previously, it was shown that 1,6-disubstituted phosphodiester-linked pyrene trimers - possessing a negatively charged backbone - assemble into sheets. Due to electrostatic interactions of the cationic and anionic species, alternated layering of the self-assembled structures is possible. The supramolecular polymers are usually preformed in their specific environment, in order to avoid problems because of different ambient conditions. Therefore, the layering on a mica substrate is done in a stepwise procedure. The structures formed by layer-by-layer self-assembly are analyzed by microscopy.

Candida antarctica Lipase B catalysis in organic, polymer and supramolecular chemistry

Dynamic enzymatic synthesis of γ-cyclodextrin using a photoremovable hydrazone template

Recent advances in DNA nanotechnology led to the development of biocompatible functional materials targeting different applications, including delivery, precise scaffolding, computation, etc. However, a standard toolbox of building blocks used by chemists is largely limited to canonical nucleobases. To further develop the functional potential of synthetic DNA systems, the use of sequence-specific DNA-polymer conjugates has emerged as an exciting alternative approach. Herein, we demonstrate the synthesis and self-assembly properties of monodisperse DNA-pyrene polyphosphate oligomers as essential units for DNA-grafted supramolecular polymers. Such systems become increasingly important for the creation of dynamic and stimuli-responsible materials. The unique self-recognition properties of oligonucleotides laid the foundation of modern DNA nanotechnology. Over the last two decades, man-made DNA assemblies led to the development of numerous biocompatible functional materials, including drug carriers, nanorobots, and scaffolding platforms. A standard toolbox of building blocks used by chemists in the field is largely limited to four units – A, G, C, T. To further develop the functional potential of synthetic DNA systems, the use of DNA-chromophore conjugates emerges as an exciting approach. Following our previous findings, we demonstrate herein the synthesis of the DNA-pyrene oligomers and the temperature-induced self-assembly of these conjugates into DNA-grafted supramolecular polymers. (see Figure 1). Such systems become increasingly important for the creation of dynamic and stimuli-responsible materials targeting various applications such as delivery and precise scaffolding.

Inspired by the vast array of assemblies present in nature, supramolecular chemistry has attracted significant attention on account of its diverse supra-structures, which include micelles, vesicles, and fibers, in addition to its extensive applications in luminescent materials, sensors, bioimaging, and drug delivery over the past decades. Supramolecular polymers, which represent a combination of supramolecular chemistry and polymer science, are constructed by non-covalent interactions, such as host-guest interactions, hydrogen bonding, hydrophobic or hydrophilic interactions, metal-ligand interactions, π-π stacking, and electrostatic interactions. To date, numerous host-guest recognition systems have been reported, including crown ethers, cyclodextrins, calixarenes, cucurbituril, pillararenes, and other macrocyclic hosts. Among them, crown ethers, as the first generation of macrocyclic hosts, provide a promising and facile alternative route to supramolecular polymers. In addition, the incorporation of fluorophores into supramolecular polymers could endow them with multiple properties and functions, thereby presenting potential advantages in the context of smart materials. Thus, this review focuses on the fabrication strategies, interesting properties, and potential applications of fluorescent supramolecular polymers based on crown ethers. Typical examples are presented and discussed in terms of three different types of building blocks, namely covalently bonded low-molecular-weight compounds, polymers modified by hosts or guests, and supramolecular coordination complexes.

The work presented in this thesis entails the synthesis and characterisation of new iron(II) tris(dioxime) clathrochelate complexes functionalised with apical capping groups suitable for supramolecular applications, and their subsequent incorporation into self-assembled structures. These complexes possess a number of desirable characteristics as supramolecular building blocks, such as their synthetic accessibility and broad scope for structural and functional modification, which are highlighted throughout. Chapter 1 introduces certain key concepts and background literature relevant to this thesis. Clathrochelate-based 4,4'-bipyridyl metalloligands are presented in Chapter 2. Simple complexes bearing 4-pyridyl capping groups were prepared from commercially available starting materials and their potential as building blocks for supramolecular architectures demonstrated through the preparation of a discrete molecular square and a 3D coordination polymer (CP). Furthermore, bent and extended linear 4,4'-bipyridyl metalloligands up to 5.4 nm in length were prepared by two distinct homologation strategies. Clathrochelate-based 3,3'-bipyridyl metalloligands are presented in Chapter 3. Simple complexes were prepared from commercially available starting materials and subsequently combined with Pd(II) ions to give unprecedented [Pd6L12]12+ octahedral cage structures by coordination-driven self-assembly. These assemblies were shown to bind the large, non-coordinating anion BPh4- in aqueous solvent mixtures. Furthermore, linear and bent bis(clathrochelate) complexes bearing 3-pyridyl apical capping groups were also prepared by a one-pot approach. Clathrochelate complexes bearing carboxylic acid-functionalised capping groups are presented in Chapter 4. Ditopic and tetratopic metalloligands were synthesised in good yields. A heterometallic 2D CP was prepared through combination of Zn(II) ions with a clathrochelate complex functionalised with phenyl-4-carboxylic acid groups and the structure of this material determined by single crystal X-ray diffraction. Clathrochelates bearing functional groups suitable for imine-based self-assembly are presented in Chapter 5. Complexes functionalised with amine or aldehyde moieties as well as complexes functionalised with 3-pyridine-4-carboxaldehyde groups, were prepared in either one or two steps. Preliminary investigations into the preparation of metal-organic cage architectures through a well-established subcomponent self-assembly approach combining imine-based DCC and coordination-driven self-assembly gave promising results, including the synthesis of an [Fe4L6]8+ cage, which was characterised by single crystal X-ray crystallography.

Orthogonal self-assembly represents a useful methodology to construct supramolecular polymers with AA- and AB-type monomers, as commonly used for covalently linked polymers. So far, the design of such monomers has relied heavily on three-dimensional macrocycles, and the use of two-dimensional shape-persistent macrocycles for this purpose remains rather rare. Here, we demonstrate a dimerization motif based on a hydrogen-bonded macrocycle that can be effectively applied to form orthogonal supramolecular polymers. The macrocycle-mediated connectivity was confirmed by single-crystal X-ray diffraction, which revealed a unique 2:2 binding motif between host and guest, bridged by two cationic pyridinium end groups through π-stacking interactions and other cooperative intermolecular forces. Zinc ion-induced coordination with the macrocycle and a terpyridinium derivative enabled orthogonal polymerization, as revealed by 1H NMR, DLS, and TEM techniques. In addition, viscosity measurements showed a transition from oligomers to polymers at the critical polymerization concentration of 17 μM. These polymers were highly concentration-dependent. Establishing this new dimerization motif with shape-persistent H-bonded macrocycles widens the scope of noncovalent building blocks for supramolecular polymers and augurs well for the future development of functional materials.

Owing to the mastery exhibited by Nature in integrating both covalent and noncovalent interactions in a highly efficient manner, the quest to construct polymeric systems that rival not only the precision and fidelity but also the structure of natural systems has remained a daunting challenge. Supramolecular chemists have long endeavored to control the interplay between covalent and noncovalent bond formation, so as to examine and fully comprehend how function is predicated on self-assembly. The ability to reliably control polymer self-assembly is essential to generate "smart" materials and has the potential to tailor polymer properties (i.e., viscosity, electronic properties) through fine-tuning the noncovalent interactions that comprise the polymer architecture. In this context, supramolecular polymers have a distinct advantage over fully covalent systems in that they are dynamically modular, since noncovalent recognition motifs can be engineered to either impart a desired functionality within the overall architecture or provide a designed bias for the self-assembly process. In this Account, we describe engineering principles being developed and pursued by our group that exploit the orthogonal nature of noncovalent interactions, such as hydrogen bonding, metal coordination, and Coulombic interactions, to direct the self-assembly of functionalized macromolecules, resulting in the formation of supramolecular polymers. To begin, we describe our efforts to fabricate a modular poly(norbornene)-based scaffold via ring-opening metathesis polymerization (ROMP), wherein pendant molecular recognition elements based upon nucleobase-mimicking elements (e.g., thymine, diaminotriazine) or SCS-Pd(II) pincer were integrated within covalent monofunctional or symmetrically functionalized polymers. The simple polymer backbones exhibited reliable self-assembly with complementary polymers or small molecules. Within these systems, we applied successful protecting group strategies and template polymerizations to enhance the control afforded by ROMP. Main-chain-functionalized alternating block polymers based upon SCS-Pd(II) pincer-pyridine motifs were achieved through the combined exploitation of bimetallic initiators and supramolecularly functionalized terminators. Our initial design principles led to the successful fabrication of both main-chain- and side-chain-functionalized poly(norbornenes) via ROMP. Utilizing all of these techniques in concert led to engineering orthogonality while achieving complexity through the installation of multiple supramolecular motifs within the side chain, main chain, or both in our polymer systems. The exploitation and modification of design principles based upon functional ROMP initiators and terminators has resulted in the first synthesis of main-chain heterotelechelic polymers that self-assemble into A/B/C supramolecular triblock polymers composed of orthogonal cyanuric acid-Hamilton wedge and SCS-Pd(II) pincer-pyridine motifs. Furthermore, supramolecular A/B/A triblock copolymers were realized through the amalgamation of functionalized monomers, ROMP initiators, and terminators. To date, this ROMP-fabricated system represents the only known method to afford polymer main chains and side chains studded with orthogonal motifs. We end by discussing the impetus to attain functional materials via orthogonal self-assembly. Collectively, our studies suggest that combining covalent and noncovalent bonds in a well-defined and precise manner is an essential design element to achieve complex architectures. The results discussed in this Account illustrate the finesse associated with engineering orthogonal interactions within supramolecular systems and are considered essential steps toward developing complex biomimetic materials with high precision and fidelity.

ConspectusIn recent years, multicomponent self-assembly has emerged as a pivotal strategy in supramolecular chemistry, enabling the construction of artificial systems with enhanced functionalities that surpass those of individual molecular components. These assemblies have garnered significant interest due to their potential applications in molecular recognition, catalysis, and biomedical engineering. However, achieving precise control over the assembly process remains a significant challenge, as increased structural complexity often results in thermodynamic mixtures, limiting their practical applications. In this context, metal-coordination-driven multicomponent self-assembly has emerged as a promising strategy, as the moderate strength and good directionality of metal-ligand bonds ensure the formation of discrete supramolecular architectures with well-defined structures and geometries. Notably, the integration of pyridyl and carboxylate donors with cis-Pt(II) nodes offers an effective approach for constructing multicomponent metallacages. This method is particularly attractive due to its ability to enable precise heteroleptic assembly, along with the accessibility and tunability of the ligands, which impart desirable photophysical properties and potential anticancer activities.This Account provides a comprehensive overview of our work on the design, preparation, and functionalization of multicomponent metallacages via the heteroleptic assembly of pyridyl and carboxylate ligands with cis-Pt(II) nodes. By strategically tailoring the ligand structures and adjusting the number of coordination sites, we have successfully constructed multicomponent metallacages with diverse geometries, including tetragonal and hexagonal prisms, triple-cavity ladders, truncated octahedra, and cyclic bis[2]catenanes, etc. In particular, we highlight the use of functional pyridyl ligands, such as tetraphenylethylene, hexaphenylbenzene, perylene diimide, and porphyrin derivatives, to explore the applications of these metallacages. Tetraphenylethylene and hexaphenylbenzene derivatives are propeller-shaped molecules with aggregation-induced emission properties, enhancing the emission of the metallacages both in solution and the solid state. Perylene diimide derivatives, with their electron-deficient, planar conjugated structures, contribute to strong emission in solution, thereby improving the host-guest chemistry and luminescent properties of the metallacages. Porphyrin derivatives, owing to their planar structures and excellent photosensitivity, endow the metallacages with significant photocatalytic capabilities. Additionally, other ligands, such as phenanthroline and triazine derivatives, have been employed to impart antibacterial and SO2 adsorption properties, respectively, to the metallacages. Furthermore, metallacage-cross-linked supramolecular networks exhibit enhanced mechanical properties and superior processability, making them promising candidates for functional materials. At the conclusion of this Account, we address the challenges and future perspectives in the development of multicomponent metallacages. We believe that ongoing research in this field will significantly advance the understanding of metal-coordination chemistry, supramolecular chemistry, and the development of functional supramolecular materials.